Rhodium-triarylphosphine complexes are commercially advantageously used as catalysts for the hydroformylation of olefins.
These rhodium-triarylphosphine complexes are chemically very stable in the presence of free triarylphosphine which is present in large excess relative to rhodium. Therefore, they have advantages in that a catalyst liquid can be separated from reaction product by distillation, returned to a reaction zone, and reused, and in that the reaction can be performed continuously while separating the reaction product by distilling away from the reaction zone by gas stripping and allowing the catalyst liquid to remain in the reaction zone. In such hydroformylation reactions of olefins, however, various high boiling by-products are formed and accumulated. In carrying out the reactions continuously, therefore, it is necessary to extract a postion of the catalyst liquid continuously or intermittently from the reaction zone.
The thus-extracted catalyst liquid contains expensive rhodium and, therefore, efficient recovery of rhodium from the extracted catalyst liquid is very important from an economical standpoint. In recovering rhodium from the extracted catalyst liquid, it is desirable to recover it in the form of a complex which is active for the hydroformylation of olefins.
Heretofore known methods of separating and recovering rhodium from the extracted catalyst liquid include a decomposition method using peroxide compounds as described in U.S. Pat. No. 3,547,964 and Japanese Patent Application (OPI) No. 63388/1976 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and a method comprising reducing aldehydes contained in high boiling by-products as described in U.S. Pat. No. 3,560,539.
The decomposition method using peroxide compounds comprises treating the extracted catalyst liquid with an aqueous solution of an acid, such as nitric acid, and a peroxide compound, and after decomposition of excess peroxide compound by heating, treating an aqueous phase resulting from the above treatment with carbon monoxide under pressure in the presence of an organic solvent and a complex-forming substance, such as triphenylphosphine, to obtain the desired rhodium complex in the organic solvent phase.
This decomposition method, however, gives rise to the problem of corrosion of apparatus material since it uses acids. Furthermore, the decomposition method suffers from disadvantages in that since the recovered rhodium complex-containing organic solvent contains sulfate ions (SO.sub.4.sup.2-) or chloride ions (Cl.sup.-), and the sulfur (S) and chlorine (Cl) poison the rhodium complex catalyst, it is necessary to apply a neutralization treatment with an alkali in the reuse of the recovered catalyst to remove the sulfur and chlorine.
In accordance with the method comprising reducing aldehydes in high boiling by-products, the extracted catalyst liquid is brought into contact with (1) alkali metal aluminum hydrides or alkali metal borohydrides, or (2) hydrogen in the presence of a solid hydrogenation catalyst, to reduce 75% of more of the carbonyl groups in the extracted catalyst liquid into hydroxyl groups, thereby modifying the extracted catalyst liquid so that the rhodium complex is sparingly soluble therein, whereby the rhodium complex is crystallized out, and separated and recovered.
This method needs the use of hydride reducing agents as in (1), or the use of hydrogen in the presence of solid hydrogenation catalysts as in (2) for the reduction of carbonyl group-containing compounds in the extracted catalyst liquid into the corresponding alcohols. When the rhodium complex crystallized out by the method is reused for the hydroformylation of olefins, it is necessary to separate the crystallized rhodium complex from the reducing agent in the case of (1) above. Also, in the case of (2) above, it is necessary to separate the crystallized rhodium complex from the solid hydrogenation catalyst, and furthermore, when the catalytic activity of the hydrogenation catalyst is reduced, regeneration treatment of the hydrogenation catalyst is needed. Moreover, the rhodium complex crystallized out by the strong reduction treatment as in (1) and (2) above does not always have enough activity for the hydroformylation of olefins.
As a result of extensive investigations to develop a method of efficiently recovering a rhodium complex from an extracted catalyst liquid for hydroformylation of olefins by a simplified method and, furthermore, as a complex which is active for the hydroformylation reaction, it has been found that when the extracted catalyst liquid for hydroformylation of olefins is brought into contact with hydrogen in the presence of an alcohol containing 1 to 8 carbon atoms and water, modified rhodium-triarylphosphine complexes in which a hydrogen atom is coordinated are precipitated.